1. Field of the Invention
This invention relates to piezoelectric type pickup cartridges.
2. Description of the Prior Art
It has been known for some time that piezoelectric elements may be used as the transducing medium in pick up cartridges such as used in stereophonic systems for playing recorded discs. The older systems used relatively brittle elements such as a Rochelle salt crystal or a porcelain substance, like barium titanate. These piezoelectric elements were made thick to compensate for their brittleness, but the thickness reduced the responsiveness of the piezoelectric to vibrations of the stylus.
Several developments have enabled the construction of piezoelectric pick up cartridges with thin piezoelectric elements. One development is the semiconductor thin film peizoelectric pick-up cartridge of the type shown in Oda et al U.S. Pat. No. 3,673,354. The other development is the high polymer piezoelectric pick up cartridge which uses as the piezoelectric element a flexible high polymer material such as polyvinylidene fluoride resin film. In the latter type of cartridges the best results can be achieved if the high polymer material can be subjected to bending torques in response to stylus vibration, while at the same time being free from local deformation at the interface between the piezoelectric element and the pressing member which presses on the piezoelectric element. However the prior art designs result in a very small contact area between the pressing member and the piezoelectric element. Examples are found in the Oda et al patent, mentioned previously, and in de Vries et al U.S. Pat. No. 3,514,550. Such designs, when used with high polymer piezoelectric elements can result in local deformations at the points of contact, which deformations prevent faithful reproduction of sound recorded on the disc in the form of vibrations. This same problem is applicable to all thin flexible piezoelectric elements.